1. Field of the Invention
The present invention relates to power converters. In particular, the present invention relates to pulse-width-modulated (PWM) converters.
2. Discussion of the Related Art
Generally, at a higher power level, a continuous-conduction-mode boost converter is a preferred implementation of a front-end converter with active input-current shaping. The output voltage of such a boost input-current shaper is relatively high, since the DC output voltage of the boost converter must be higher than the peak input voltage. Due to this high output voltage, a fast-recovery boost rectifier is required. At a high switching frequency, a fast-recovery rectifier produces a significant reverse-recovery-related loss when it is switched under a "hard-switching" condition. (See, for example, "New fast recovery diode technology cuts circuit losses, improves reliability," by Y. Kersonsky, M. Robinson, and D. Gutierrez, Power Conversion & Intelligent Motion (PCIM) Magazine, pp. 16-25, May 1992.) As a result, "hard-switched", boost input-current-shapers are operated at relatively low switching frequencies to avoid a significant deterioration of their conversion efficiencies. Using a soft-switching technique, the switching frequency and, therefore, the power-density of the boost, front-end converter can be increased.
So far, a number of soft-switched boost converters and their variations have been proposed. Some examples of soft-switched boost converters are disclosed in the following references: (a) "High efficiency telecom rectifier using a novel soft-switched boost-based input current shaper," ("Streit") by R. Streit, D. Tollik, International Telecommunication Energy Conf. (INTELEC) Proc., pp. 720-726, October 1991; (b) U.S. Pat. No. 5,418,704 ("Hua et al."), entitled "Zero-Voltage-Transition Pulse-Width-Modulated Converters" to G. Hua, F.C. Lee, issued May 23, 1995; (c) U.S. Pat. No. 5,446,336, entitled "Boost Converter Power Supply with Reduced Losses, Control Circuit and Method Therefor" ("Bassett et al.") to J. Bassett and A. B. Odell, issued Aug. 29, 1995; and (d) U.S. Pat. No. 5,736,842 ("Jovanovic"), entitled "Technique for reducing rectifier reverse-recovery-related losses in high-voltage, high-power converters," to M. Jovanovic, issued Apr. 7, 1998.
Each of the references (a)-(d) above discloses an auxiliary active switch operating together with a few passive components (e.g., inductors and capacitors), thus forming an active snubber that is used to control the rate of change of rectifier current (di/dt) and to create conditions for zero-voltage switching (ZVS) of the main switch and the rectifier. Active snubbers are described, for example, in "Switched snubber for high frequency switching," ("Harada et al.") by K. Harada, H. Sakamoto, IEEE Power Electronics Specialists' Conf (PESC) Rec., pp. 181-188, June 1990. FIGS. 1-3 show the soft-switched boost circuit introduced in Hua et al., Bassett et al., and Jovanovic, respectively.
The boost converter circuits proposed in Streit and Hua et al. use a snubber inductor connected to the common node of the boost switch and the rectifier to control the rate of change of rectifier current (di/dt). As a result of the snubber-inductor location, the main switch and the rectifier in the circuits proposed in Streit and Hua et al. possess minimum voltage and current stresses. In addition, the boost switch closes and the rectifier turns off under zero-voltage (soft-switching) conditions. However, the auxiliary switch operates under "hard" switching conditions, as it is closed while its voltage is equal to the output voltage, and subsequently opened while carrying a current greater than the input current.
In the circuits of Bassett et al. and Jovanovic, the rate of change of rectifier current is controlled by a snubber inductor connected in series with the boost switch and the rectifier. Because of this placement of the inductor, the voltage stress of the main switch is higher than that of the circuits described in Streit and Hua et al. This increased voltage stress can be minimized by a proper selection of the snubber-inductance value and the switching frequency, as taught in Jovanovic. Both the boost and the auxiliary switches in the circuits in Bassett et al. and Jovanovic, as well as the boost rectifier, operate under ZVS conditions.
The major deficiency of the boost converters described in Streit and Hua et al. is a severe, undesirable resonance between the output capacitance C.sub.OSS of the auxiliary switch and the resonant inductor. The undesirable resonance occurs after the auxiliary switch is opened and the snubber inductor current falls to zero and adversely affects the operation of the circuit and must be eliminated. For example, in the circuit introduced in Hua et al., the resonance is eliminated by connecting a rectifier and a saturable inductor in series with the snubber inductor, as shown in FIG. 1, which degrades the conversion efficiency and increases both the component count and the cost of the circuit.
The circuits described in Bassett et al. and Jovanovic require an isolated (high-side) gate drive, which increases circuit complexity and cost. Also, the circuit introduced in Jovanovic requires noise-robust gate-drive timing since accidental transient overlapping of the main and auxiliary switch gate drives may lead to a fatal circuit failure resulting from the relatively large transient current through the series connection of the simultaneously-conducting main and auxiliary switches. (The circuit introduced in Bassett et al. does not suffer from the overlapping gate-drive problem because it requires overlapping gate drive for proper operation.)